1. Field of the Invention
The present invention relates to an ink jet printer and more particularly to such a printer which is adapted to print characters in graphic patterns in multiple colors.
2. The Prior Art
Multicolor printers are known which provide several separate ink reservoirs each containing ink of a different color. See German Pat. No. OS 29 25 812. An optically suitable multicolor representation is possible using inks of the primary colors magenta, cyan and yellow. It is expedient to use black as a fourth color in a multicolor printer.
A major element of the multicolor printer is the print head. The print head has several ink channels arranged which are connected to the various ink reservoirs, and from which individual ink droplets are ejected, under the control of piezoelectric drive elements associated with the individual channels.
The print head and the recording carrier are caused to move relative to each other, and droplets of appropriate color are ejected to various positions on the recording carrier, with the result that multicolor characters or patterns can be represented in dot matrix fashion. The lines and rows of the dot matrix are determined by the relative movement between the recording carrier and the print head, as well as by the frequency of ejection of the ink droplets. In order to produce mixed colors, a given location of the recording carrier is repeated by printed with ink droplets of different primary colors. In this fashion, using the primary color cyan, magenta and yellow, the secondary colors of red, green and blue can be represented.
It frequently occurs with multicolor presentations of characters or patterns that, when different colors meet, the contours are unsharp and the color appears to have a dirty or blurred optical impression. The reason for this is that, in the formation of the mixed colors, several ink droplets of different colors are successively applied to the same point of the recording carrier. With each application, the diameter of the dot is enlarged so that dots or image points of varying diameter result on a recording carrier. This creates the impression that the colors are run together or blurred, an overall unsharp effect. This also applies to the representation of graphics.
It is known, using multicolor printers, to employ a method wherein the image elements or pixels are defined individually in which each exhibit one or more picture points. Each image point is printed by one or more ink droplets. In order to obtain a good color gradation, or desired color mixtures, the individual image points of an image element are filled up according to a specific rule, sometimes referred to as the Dither-matrix. Different image point diameters, such as result from the spraying of individual droplets on top of each other, lead to uncontrolled overlapping at the edges and therefore to erroneous colors in the overlapping regions within a picture element.
It is therefore desirable to provide an apparatus and method for a multicolor ink jet printer to represent both the contours of the characters and patterns more clearly and sharply, as well as to improve the color contrast and color saturation which is achieved.